1. Field of the Invention
The present invention relates to an optical modulation element using a material having refractive anisotropy and, more particularly, to an optical modulation element using ferroelectric liquid crystals (to be referred to as FLCs hereinafter) as the material. This optical modulation element is suitably used in a display unit for displaying character and graphic images.
2. Related Background Art
In an optical modulation element using a ferroelectric liquid crystal, a scheme for producing a bistable state by using surface actions of two parallel plates having a gap (e.g., 1 to 2 .mu.m) in which a liquid crystal layer is sealed therein is promising in a variety of applications due to its high-speed response and good memory properties.
The bistable ferroelectric liquid crystal element exhibits a bistable state in a liquid crystal molecular axis inclined by a predetermined angle from an axial direction (rubbing direction or the like) of the inner alignment surfaces (formed by rubbing or the like) sandwiching a liquid crystal layer. This angle is called a cone angle (to be referred to as .theta..sub.c hereinafter).
When a voltage is applied to the element in a direction perpendicular to the liquid crystal layer surface, the state of the ferroelectric liquid crystal transits from one stable state to the other stable state. This change corresponds to rotation of an index ellipsoid of a material having diffractive anisotropy about its major axis through an angle 2.theta..sub.c within the liquid crystal layer surface. Strictly speaking, the molecular axis of the liquid crystal is not necessarily aligned with the major axis of the index ellipsoid. However, these axes are regarded to be aligned with each other for the illustrative convenience. When polarized light is incident on the ferroelectric liquid crystal element having a thickness corresponding to the action of a .lambda./2 plate, polarization/rotation actions for the incident polarized light by the two stable states are different from each other by 4.theta..sub.c. When the ferroelectric liquid crystal element is sandwiched between the polarizing elements (polarizing plates or the like) of crossed or parallel Nicol prisms, the ON/OFF ratio (transmittance ratio and contrast) of the amount of transmitted light components in the two stable states has a maximum value when 4.theta..sub.c =90.degree. (.theta..sub.c =22.5.degree.).
FIG. 12 shows a ferroelectric liquid crystal element in which polarizing elements are arranged in the form of crossed Nicol prisms. Polarized light Ein having passed through a polarizer (not shown) is incident on a liquid crystal layer 125 having an action corresponding to a .lambda./2 plate. In this case, a molecular axis 1255 of the liquid crystal of the liquid crystal layer 125 in one stable state is aligned with the polarization direction of the incident light. An analyzer (not shown) is arranged perpendicularly to the polarizer. A molecular axis 1256 corresponds to the other stable state. The molecular axes 1255 and 1256 of the liquid crystal respectively have cone angles of +.theta..sub.c and -.theta..sub.c from an alignment axis 1254 formed by rubbing or the like. When the liquid crystal molecules are set in the state corresponding to the axis 1255, the polarization direction of the incident polarized light is not changed and the incident polarized light is entirely shielded by the analyzer, thereby expressing black. On the other hand, when the liquid crystal molecules are set in the state corresponding to the axis 1256, the polarization direction is rotated through 4.theta..sub.c, and the ratio of light transmitted through the analyzer is given as: EQU sin.sup.2 (4.theta..sub.c)
thereby expressing a white state.
When the polarizing elements are arranged in the form of crossed Nicol prisms, the liquid crystal molecules in the state corresponding to the axis 1255 express the white state, and the liquid crystal molecules in the state corresponding to the axis 1256 express the black state.
The cone angle .theta..sub.c of the bistable ferroelectric liquid crystal has considerably high temperature dependency. For this reason, even if the bistable ferroelectric liquid crystal is arranged at a given temperature, as shown in FIG. 12, the polarization direction of the incident light is deviated from the direction of the molecular axis 1255 of the liquid crystal in one stable state at another temperature. For this reason, a sufficiently dark black state cannot be obtained in the form of crossed Nicol prisms, thus degrading the image contrast.
On the other hand, when the polarizer and the analyzer are arranged in the form of parallel Nicol prisms, the transmittance in the white state is decreased.